1. Field of the Invention.
The invention relates to a method and a device for microwave-heating prepared meals sealed in trays, in which the trays are placed in an autoclave into which microwaves from microwave generators are coupled and in which the trays are moved back and forth while they are being processed.
2. Description of the Related Art.
Such a method and such a device are known from EP 0 347 623. The approach attempted in that case is to bring about the necessary heating of the foods deposited in portions on trays by diffusely filling the upper interior space of the autoclave with microwave energy, for which purpose microwave generators in the form of magnetrons are arranged around the upper half of the autoclave such that waveguides leading from the microwave generators into the autoclave are directed at the upper surfaces of the foods deposited on the trays and other waveguides are directed from the side of the autoclave with their decoupling openings confronting each other, at a relatively large distance from the undersurfaces of the trays.
In plants for the manufacture and processing of prepared meals sealed in trays, the portions deposited ready for consumption on trays and composed of meat and gravy, potatoes or noodles, and vegetables must be sterilized and/or pasteurized by heating in microwave ovens before being deep-frozen for storage. In this connection, care must be taken that these portioned, temperature-sensitive products do not exceed or fall below certain temperatures while being processed in heating systems. The microwave ovens that are used in these plants, and that usually accommodate a hundred or more trays, are equipped for this purpose with microwave generators, often hundreds of them, in the form of magnetrons.
The microwave ovens used for this purpose are autoclaves in which the pressure can be increased to such a high level that water present in the foods to be heated does not boil even at temperatures of 125° C. or more, and the tray seals, which are usually of plastic sheet, do not burst. The products disposed on a tray must be heated as evenly and steadily as possible in order to prevent overheating and burning (hot spots) at individual sites that are preferentially toward the edges, and, conversely, to prevent unheated or poorly heated sites (cold spots) in the middle, even though the energy absorption and heating characteristics of the individual products differ. For example, microwaves penetrate different products to different depths. While they have high depths of penetration in starch products such as potatoes and noodle dishes and therefore heat these foods evenly with greater speed, the depths of penetration in meat are much smaller.
The approach used to couple microwave energy into known microwave ovens is less than satisfactory. If the waveguides feeding the microwaves into the autoclave conduct the microwave energy into the autoclave from below, they are designed and arranged so that the microwaves fill the interior of the autoclave diffusely, with the objective of producing uniform heating of the material that is to be heated. This objective has not been achieved in practice. The waveguides conducting the microwave energy into the autoclave have therefore been run from the top down to immediately above the product to be heated and decoupled in that location, just above the upper surfaces of the products to be heated, with the end result that the upper surface and the headspace of the sealed tray were heated intensely and in that area the products to be heated were brought to a boil or actually caught fire and even burst the tray seal, but owing to low depths of penetration the central and lower regions were not heated satisfactorily. This is also the case when microwave energy is coupled out both below and above the products to be heated.
In addition, it was found that when diffuse filling of the interior of a microwave oven was used, the energy being coupled in tended to be extinguished by interference, so that “cold spots” kept occurring in which the products to be heated were not sterilized and pasteurized and therefore constituted a threat to human health.
From EP 0 344 408, it is known, in the microwave heating of food portions resting on trays, to keep the air temperature inside the microwave oven the same as or slightly higher than the final temperature of the foods being heated. This has proved to be inexpedient, since the outer surface of the food to be heated is raised to a higher temperature than the interior of the food. The environmental temperature of foods undergoing microwave heating can certainly be set higher than the initial [sic] temperature, but heating the outer and upper surfaces of the foods more vigorously and intensely than their interior must be avoided if at all possible.